The invention relates to an apparatus and method for purging charged powder particles from a mixture of said powder particles and ferromagnetic carrier particles. Carrier particles comprising a ferromagnetic core surrounded by a thin coating are used for instance in applications such as laser printing and powder coating of substrates to transport powder particles such as toner particles and/or powdered paint particles towards a substrate on which the powder particles are to be applied. When changing powder types, for instance when changing from a black toner to a yellow toner, the batch of carrier particles used must be changed as well to avoid mixing of the earlier color with the latter color on the substrate. To reduce the number of batches of carrier particles that must be kept at hand, a number of cleaning methods have been proposed in the art. Many of these comprise cleaning the carrier particles using liquids and solvents, which are environmentally unfriendly and require the carrier particles to be dried for some time before they can be reused.
U.S. Pat. No. 6,751,430 describes a toner purging apparatus for cleaning carrier particles from charged toner particles without using a liquid. The purging apparatus comprises a roller with an outer surface to which toner laden carrier particles are attracted by means of a magnetic core placed within the roller and adapted for counter rotation with respect to the surface. A coronode wire, at a voltage having a polarity opposite to that of the charged toner particles, is arranged close to the outer surface of the roller and adapted for detoning the magnetic carrier particles by repelling the toner particles away from their laden relationship with the magnetic carrier particles and away from the coronode wire and therefore onto the outer surface. The carrier particles are then skived off the outer surface and collected in a sump.
A drawback of the known apparatus is that as the toner is separated from the carrier particles by discharge of a coronode wire, the known apparatus is not suitable for quickly purging large amounts of toner from carrier particles. Moreover, a coronode wire typically generates sparks and/or ozone, and must often be replaced due to wear.
It is an object of the present invention to provide a powder purging apparatus and method solving at least one of the drawbacks mentioned above.